1. Field of the Invention
This invention relates to nickel-iron-type perovskite materials for an air electrode used for a solid oxide fuel cell, and particularly relates to the materials which are capable of improving the reliability of the solid oxide fuel cell and also capable of improving generation efficiency of electricity of this type of fuel cell.
This application is based on Patent Application No. Hei 9-356061 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
A fuel cell is a type of gas-electric cell capable of operating for long periods of time, by, on one hand, supplying oxygen or air to the cathode and supplying hydrogen or a hydrocarbon to the anode and, on the other hand, by continuously removing the reaction product (such as H.sub.2 O or CO.sub.2) from the fuel cell. In particular, from the point of view of effective utilization of energy, the fuel cell has a high conversion efficiency of energy, since the fuel cell is free from the thermodynamic restraints of the Carnot efficiency, so that the fuel cell is expected to be advantageous in environmental protection.
Of the various types of fuel cells, recently, solid oxide type fuel cells have been investigated extensively and, in particular, the solid oxide cells using an ionic conductor of oxygen are attracting attentions.
A tube-type cell unit as a representative example of the solid oxide fuel cell has the structure which is schematically shown in FIG. 1. This tubular cell unit, as shown in FIG. 1, is formed by a cylindrical porous substrate of the air electrode 1, a solid electrolyte 2 and a fuel electrode 3 disposed on the opposite sides of the air electrode 1, and an inter-connector for connecting cell units to each other in the fuel cell. This structure is advantageous in the ease of constructing a sturdy cell and attaining a gas-tight structure. However, a drawback is found that the length of the circuit of the electric current to flow is too long, causing energy loss.
At present, YSZ (Yttrium Stabilized Zirconia) or SASZ (Scandium Aluminum Stabilized Zirconia) are the most promising materials as the solid electrolyte. Although many materials are examined for the air electrode, a manganese-type oxide with a perovskite type structure such as La.sub.0.8 Sr.sub.0.2 MnO.sub.3 is now being investigated. However, due to its low electric conductivity, the resistance loss of energy of the above perovskite material degrades the power generation efficiency of the fuel cell. Thus, a material with high electronic conductivity is required.
In general, it is necessary to operate a fuel cell at a temperature of 1,000.degree. C. at present. This is because the fuel cell, composed of the air electrode, the fuel electrode, and the solid electrolyte, cannot generate sufficient power effectively at lower temperatures than 1,000.degree. C. However, since high temperatures such as 1,000.degree. C. cause, for example, sintering of the fuel electrode, which degrades the power generation efficiency and which restricts the development of the fuel cell. Thus, it is desired to reduce the operation temperature of the fuel cell to 800.degree. C.
In order to reduce the operation temperature of fuel cells, it is necessary to take many measures. An important point is to improve the electronic conductivity of the air electrode materials such as perovskite-type ceramics.
Among perovskite-type ceramic materials, some example such as La(Sr)CoO.sub.3 is known as a conductive material with high electric conductivity. However, a problem arises that cracks may occur at an interface with the solid electrolyte, since this material has an higher expansion coefficient about two-times than that of the solid electrolyte of YSZ or SASZ.
That is, the air electrode material is required to have an approximately equal thermal expansion coefficient to that of the solid electrolyte of YSZ or SASZ. This is required to avoid cracks in the electrolyte caused by a stress originated by mismatching of thermal expansion coefficients between the solid electrolyte and the air electrode in temperature cycles between room temperature and the operation temperature of 1,000.degree. C.
As hereinabove described, there are two essential problems for the conventional tubular cell unit of the fuel cell, one of which is a problem concerning the reliability of the cell unit in term of cracking of the solid electrolyte, and another one of which is a problem concerning the low power generation efficiency caused by the low conductivity of the air electrode.
Here, the thermal expansion coefficients of conventional materials used in a conventional solid oxide fuel cell are shown in Table 1 as a reference.
TABLE 1 ______________________________________ Thermal expansion coefficient of materials used in a conventional solid oxide fuel cell Thermal expansion coefficients Materials (.times.10.sup.6) (1/K)* ______________________________________ YSZ 10.0 SASZ 10.0 La.sub.0.8 Sr.sub.0.2 MnO.sub.3 12.0 YSZ-Ni Cermet (Ni: 60 mol %) 13.0 La.sub.0.8 Sr.sub.0.2 CrO.sub.3 10.0 ______________________________________ *: average thermal expansion coefficients from 25 to 800.degree. C.
As shown in Table 1, the thermal expansion coefficients of the dense solid electrolytes (YSZ and SASZ) and the inter-connector (La.sub.0.8 Sr.sub.0.2 CrO.sub.3) are identical. In contrast, a conventional material for the fuel electrode of a cermet represented by Ni-YSZ cermet and the material for the air electrode expressed by La.sub.0.8 Sr.sub.0.2 MnO.sub.3 have higher thermal expansion coefficients by 20 to 30% than that of the electrolyte of YSZ or SASZ. A 20% to 30% differences in thermal expansion coefficients between the solid electrolyte and the materials for the fuel electrode and the material for the sir electrode might be allowable because the fuel electrode is formed in a porous layer which is capable of absorbing the difference of thermal expansion. However, in order to improve the reliability of the solid oxide fuel cell, a difference in thermal expansion coefficient between the solid electrolyte and the air electrode material is desired to be restricted within 10%, even though the air electrode is formed in a porous body.
It is therefore an object of the present invention to provide a material which has a thermal expansion coefficient close to that of the solid electrolyte, and, at the same time, has a high level of electric conductivity so as not to degrade the energy generation efficiency by a high resistivity.